IMO although you've got value, his bet is probably at the going rate. Without even looking id go about 8/11 - 4/5 the Albion which by default would put the scum at about even money
I reckon you're safe there, seeing as it looks like Palace can only afford to sign players on Bosmans they aren't likely to do any better than the last two seasons. Still, they'll have a decent League One striker ready for 2012-2013.
I have done the similar bet with a work mate only for a £100.00, at first I thought it was a bit much and wanted to do £50 However he instisted that we make it £100 implying I had a lack of confidence in the Albion. I am hoping we do !! As there is £200 sat in an envelope in the work safe waiting for me.
In other stupid bet news, I made a slap-bet (the winner gets to slap the loser as hard as they possibly can) last August with a Leeds fan on who would reach the Premier League first. Yes, I was drunk at the time but we've stuck with it as it was quite funny. Suddenly not looking quite so stupid though - and looking at the link above we'd even be favourites in that one!